From a recent review in The Lancet:

More than 10 million people are incarcerated worldwide, and this number has increased by one million in the past decade.

Some of the major medical problems include:

- Mental disorders and infectious diseases are more common in prisoners
- High rates of suicide in prison
- Increased mortality on release

High risk groups among prisoners:

- women
- prisoners aged 55 years and older
- juveniles

References:
The health of prisoners. The Lancet, Volume 377, Issue 9769, Pages 956 - 965, 12 March 2011.
“A very severe physical trial”. The Lancet, Volume 377, Issue 9769, Page 876, 12 March 2011.

Source:
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